Transplantation is the preferred mode of therapy for many forms of end-stage organ disese. Success in transplantation has been built around therapeutic approaches to control the T cell-dependent process of rejection. Current therapies control rejection but cause numerous non-immune toxicities. In the past few years the concept of controlling rejection with more immuno-selective approaches by targeting T cell costimulatory pathways has shown promise in clinical trials. While very effective in many experimental rejection models, it is widely recognized that costimulation blockade approaches targeting CD28 and/or CD40 do not uniformly control rejection responses. As these strategies progress in clinical trials the need to dissect the mechanisms by which T cells can escape blockade of these pathways becomes more pressing. Over the past several years, the Programmed Differentiation Model has emerged as a new pardigm to understand T cell responses. This model is based on evidence that after a brief period of antigenic stimulation, T cells become committed to a program of autonomous clonal expansion of several rounds of cell division and differentiation into effector cells. However, T cell programs are flexible and can be altered by the initial priming conditions and by extrinsic factors during the execution of the program. Two findings of particular relevance to transplantation are recent studies indicating 1) that the initial precursor frequency of the responding population is a powerful influence on the program and that high initial CD8+ T cell frequencies can convert helper dependent responses into helper-independent and costimulation- independent responses, and 2) that both IL-2and IFNg can play critical roles during the antigen-independent expansion/differentiation phase of the CD8+ T cell program. Experimental evidence suggest that between 0.1-10% of a naive individual's T cell repertoire is capable of reacting with alloantigens, a figure that is 2-3 logs greater than the estimated precursor frequency of virus- specific T cell responses. The central hypothesis of this proposal is that variation in the initial precursor frequencies of donor-reactive CD4 and/or CDS T cells is a critical determinant in the susceptibility or resistance to CD28/CD40 costimulation blockade induced graft acceptance.